Numerous sensors that generate electronic output signals representing a sensed physical condition undergo a variation in their output impedance with signal amplitude. This is a characteristic of the sensor "on-chip" amplifiers. The fundamental problem caused by these amplitude variations is that the output signal from the on-chip amplifier will give false indications of sensed physical conditions. An explanation of this result may be given as follows.
A sensor with its on-chip amplifier (herein referred to as the "sensor") may be viewed as composed of a voltage source in series with an impedance. The output of this circuit is likely to be in the millivolt to volt range and will require further amplification with what may be characterized as a pre-amplifier with an input impedance that is generally large with respect to the sensor impedance. When the sensor impedance undergoes a variation due to signal amplitude, the sensor output will not only indicate a variation due to changes in the sensed physical condition but also, because of the variation of the sensor impedance, an error component. The signal to the pre-amplifier, then, is derived from the junction of a divider composed of the varying sensor impedance and the pre-amplifier input impedance.
While sensor errors are caused by sensor impedance variations that are dependent on signal amplitude, errors are also introduced by the characteristics of the pre-amplifier or other signal processing amplifiers that follow the sensor on-chip amplifiers. The current technology generally employs operational amplifiers because of some positive characteristics that they possess for this application, such as high open loop voltage gain, low current noise, and small inter-electrode capacitance between the inverting- and non-inverting inputs. However, the use of operational amplifiers also introduces a primary negative characteristic that causes the introduction of errors in the form of a one-pole frequency characteristic which results in a boot-straped input impedance that is not constant with frequency, i.e., the input impedance varies with the frequency of the applied signal. This results in a configuration or equivalent circuit comprised of a signal source, including a voltage source and a series source impedance that varies with signal amplitude, which, in turn, is followed by another impedance, the amplifier input impedance, that varies with signal frequency. If the amplifier input impedance could be made constant with frequency and very high relative to the sensor source impedance, the effect of variation of the source impedance on error production can be minimized. In other words, the varying sensor source impedance would have a negligible effect on the network transfer function, from voltage source to the pre-amplifier, if the pre-amplifier input impedance is made very large. The prior art recognized the existence of the error sources but the solutions provided were partial.
The prior art, when employing these sensors, e.g., a CCD array where each CCD is followed by an on-chip FET amplifier, recognized that the sensors, due to the on-chip amplifier design, have a high source impedance characteristic. To minimize the effect of the source impedance variations that occur with sensor generated voltage variations, the prior art employed buffer circuits as input stages of the signal processing amplifiers. The buffer amplifiers, for example, employed single bipolar transistors, single FETs, as well as operational amplifier followers, as previously noted. However, while the buffers could provide a very large input impedance at low frequencies, they all suffer a reduction in input impedance with increasing frequency that may become rather severe at very high frequencies.